1. Field of the Invention
The present invention relates to a frame transfer method, and more particularly to a frame transfer method employed for VPN services for realizing virtual private networks (VPN).
2. Description of Related Art
There is a VPN service proposed for forming a virtual private network (VPN) in an enterprise by connecting a plurality of the enterprise sites separated physically away from one another. In recent years, another VPN service has started. The new VPN service transfers frames according to MAC addresses, which are of the Ethernet (trademark). Each of this type networks is a comparatively small in scale and formed, for example, within an urban community and referred to as a MAN (Metropolitan Area Network).
On the other hand, there is a technique for realizing a wide ranged large scale network configured by a plurality of such the MANs. This technique is an application of the MPLS (Multi Protocol Label Switching) proposed, for example, in the IETF Draft “Encapsulation Methods for Transport of Layer 2 Frames Over IP and MPLS Networks”, draft-martini-12 circuit-encap-mpls-04.txt referred to as the conventional technique 1 and in the IETF Draft “Transport of Layer 2 Frames Over MPLS”, draft-martini-12-circuit-encap-mpls-08.txt referred to as the conventional technique 2. In those conventional techniques 1 and 2, a path referred to as a tunnel LSP (Label Switching Path) is formed in a backbone network connected to a plurality of MANs and a plurality of paths referred to as VC LSPs are formed in this path (tunnel LSP). A node located at the inlet of the back-born network that connects the MANs adds a tunnel label and a VC label to each received frame. Both of the tunnel and VC labels are identifiers of those LSPs. And, the nodes in the back-born network transfer those frames while the node located at the outlet of the back-born network processes the frames according to their VC labels.
FIG. 2 shows a block diagram of a network to which such a conventional frame transfer method applies.
Hereunder, the conventional techniques 1 and 2 will be described with reference to the block diagram of the network shown in FIG. 2. In the network shown in FIG. 2, the sites LAN-A1 and LAN-A2 of an enterprise A are connected to each other through MAN-1, MAN-3, and a backbone network that connects those MAN-1 and MAN3 respectively. The backbone network is configured by PEs (PE: Provider Edge Node) 1 to 3 located on the edge thereof and PCs (PC: Provider Core Node) 1 to 3. In the backbone network, tunnel LSPs (T-LSP2 and T-LSP4) are formed. The T-LSP2 transfers frames in the direction of PE1->PC2->PC3->PE3 while the T-LSP4 transfers frames in the opposite direction. In the T-LSP2, a VC-LSP-A1 is formed so as to transfer frames from the LAN-A1 to the LAN-A2. In the T-LSP-4, a VC-LSP-A2 is formed so as to transfer frames from the LAN-A2 to the LAN-A1. In addition, another LSP used for communications between each site of an enterprise B and each site of an enterprise C is formed in the backbone network. The LSP illustration is omitted in FIG. 2, however.
PE1 of the backbone network, when receiving a frame from the LAN-A1, adds a tunnel label that is the T-LSP2 identifier and a VC label that is the VC-LSP-A1 identifier to the frame, then transfer the frame to the PC2. The PC2, as well as the PC3 refer to the tunnel label to transfer the frame to the PE3. The PE3 then refers to the VC label to transfer the frame to a line connected to the MAN-3. Consequently, the MAN-1 and is connected to the MAN-3, thereby the VPN service of the enterprise A is realized.
Next, the problems of the conventional techniques 1 and 2 will be described with reference to the network shown in FIG. 1, which is the same as the network shown in FIG. 2. In FIG. 1 are shown only the LSPs formed among the sites of the enterprises A and B.
In the network shown in FIG. 1, the sites LAN-B1 to B4 of the enterprise B are connected to one another through the MAN-1 to MAN-4, as well as the backbone network that connects those MANs. In the backbone network, tunnel LSPs (T-LSP1 and T-LSP3) are formed. The T-LSP1 transfers frames in the direction of PE1->PC1->PE2 and the T-LSP3 transfers frames in the opposite direction. In the backbone network, other tunnels LSP (T-LSP2) and LSP (T-LSP4) are also formed. The LSP (T-LSP2) transfers frames PE1->PC2->PC3->PE3 and the LSP (T-LSP4) transfers frames in the opposite direction. In the T-LSP1, a VC-LSP-B1 is formed so as to transfer frames from the LAN-B1 to the LAN-B2. In the T-LSP3, a VC-LSP-B3 is formed so as to transfer frames in the opposite direction. In the T-LSP2, a VC-LSP-B2 is formed so as to transfer frames from the LAN-B1 to the LAN-B3 and B4. In the T-LSP4, a VC-LSP-B4 is formed so as to transfer frames in the opposite direction. In the backbone network are also formed still other LSPs; an LSP used for the communications among the sites of the enterprise A, an LSP used for communications among the sites of the enterprise C, and an LSP used for the communications between PE2 and PE3, although those LSPs are not shown in FIG. 1.
In a network configured as described above, the PE1, when receiving a frame from the LAN-B1, cannot decide to which of LAN-B2, B3, and B4 the frame should be transmitted. In other words, the PE1 cannot decide which of the tunnels (VC-LSP-B1 in the T-LSP1 and T-LSP-B2 in the T-LSP2) should be used to transfer the frame through the VC-LSP. This is also the same for the PE3, which cannot decide which of the lines connected to MAN-3 and MAN-4 should be used to transfer the frame. Consequently, the conventional techniques 1 and 2 described above cannot connect any site over three or more MANs.
On the other hand, there is a technique for connecting a site over three or more MANs. This technique enables the subject PE to learn an output line number, a tunnel LSP, and a VC-LSP in accordance with the MAC address set in each frame. Such the technique is known well as the conventional technique 3 (IETF Draft “Virtual Private Switched Network Services over an MPLS Network”, draft-vkompella-ppvpn-mpls-00.txt) and the conventional technique 4 (IETF Draft “Transparent VLAN Services over MPLS”, draft-lasserre-vkopella-ppvpn-tis-00.txt). A PE, when receiving a frame from a PC belonging to the backbone network, stores transfer information consisting of the line number of the line to which the frame is inputted, the tunnel LSP, and the VC-LSP therein in accordance with the source MAC address set in the received frame. And, the PE, when receiving a frame from a MAN node, stores transfer information consisting of the line number of the line to which the frame is inputted therein corresponding to the source MAC address set in the frame. When receiving a frame addressed to the stored MAC address, the PE transfers the frame according to the transfer information corresponding to the MAC address.
Next, the conventional technique 3 will be described in detail with reference to FIG. 1. The PE1, when receiving a frame from the terminal T7 belonging to the LAN-B3, stores the line number of the line connected to the PC2, the VC-LSP-B2, and the T-LSP2 therein in correspondence with the MAC address of the terminal T7. When the PE1 receives a frame addressed to the terminal T7 from the MAN-1, the PE1 transfers the frame according to the line number, the VC-LSP-B2, and the T-LSP2 stored therein as described above. The PE3, when receiving a frame from a terminal T7, stores the line number of the line to which the frame is inputted and the MAC address of the terminal T7 so that the line number and the MAC address are corresponded to each other. And, the PE3, when receiving a frame addressed to the terminal T7 from the PC3, transfers the frame to the line corresponding to the line number stored therein.
As described above, according to any of the conventional techniques 3 and 4, when a frame is received from a MAN, it is possible to decide to which of the remaining two or more MANs the frame should be transmitted. This is why a site can be connected over three or more MANs, thereby the problems of the conventional techniques 1 and 2 are solved.
The conventional techniques 1 to 4 described above, however, are often confronted with the following problem that will arise in construction of a large scale network that comes to include many enterprises (contractors) connected over a plurality of MANs. The conventional techniques 1 and 2 also come to be confronted with another problem that a site connected over three or more MANs as described above cannot be connected to a network that employs any of the conventional techniques 1 and 2.
Furthermore, a network that employs any of the conventional techniques 3 and 4 causes another problem to arise; the capacity of a table provided in each node to store transfer information often becomes insufficient. In other words, every PE that employs any of the conventional techniques 3 and 4 is required to learn such transfer information as output line numbers, tunnel LSPs, VC LSPs in correspondence with the MAC addresses of all the enterprises stored in the PE. For example, the PE1 shown in FIG. 1 is required to learn such the transfer information so as to make it correspond to the MAC addresses of all the terminals T1 to T11 of the enterprises A to C. And, the table provided in such a PE so as to store such transfer information is limited in capacity and due to the limited capacity of the table, the networks that employ any of the conventional techniques 3 and 4 come to be disabled to store information of many enterprises (contractors).
Under such circumstances, it is an object of the present invention to provide a network that can hold information of many more enterprises (contractors) than any conventional networks by forming nodes on the edge of the subject network, which are used to store frame transfer information corresponding to the destination address of each frame.